Genetic Screening Options in Pregnancy

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May 17, 2022 | Miri Levi | @midwifemiri

Congratulations on your pregnancy! One of the early decisions that you will need to make is whether to do genetic screening testing. Laura Rust, one of the obstetrics genetic counselor's at Mayo Clinic Rochester shares some of the most important information to consider when deciding if you’d like to complete fetal genetic screening during your pregnancy.

Genetic screening is an option for all pregnant patients. Some patients think that they won’t qualify for genetic testing unless they are over 35 years old. While the risk of having a baby with a genetic condition increases with age, all women have some risk of a genetic condition (called a baseline risk). Even at age 40, the baseline risk of Down’s syndrome is only 1%.

Some expectant parents may be anxious because they know someone who has had a baby with a genetic condition or themselves have had that experience. A normal test can give them some peace of mind. Other patients find genetic testing helpful in planning and preparing for their baby’s arrival if there is an unexpected diagnosis of a genetic condition. Sometimes having a known diagnosis will change what testing is needed during your pregnancy. Or special arrangements might need to be made for the arrival of your little one. Although some genetic conditions can be suspected based on ultrasound findings, others may have no noticeable signs on ultrasound.

Families have different feelings about what they would do with the news of a diagnosis of a genetic condition. This is a very personal decision for families to make. Our goal is to give you the information you need to make the right decision for your family.

Screening tests are just screening tests. This means that the test will tell you if your baby is at a higher risk for a certain condition, but it does not tell you for sure if your baby has that genetic condition. If a screening test shows an increased risk, this usually prompts additional testing to see if the screening test is accurate (called confirmatory testing).

One of the most common chromosomal conditions that is screened for in pregnancy is Down syndrome, also known as Trisomy 21. Many prenatal genetic screening tests also assess the risk of conditions such as Trisomy 18 (Edwards syndrome) or Trisomy 13 (Patau syndrome). These are severe chromosomal conditions but fortunately less common.

There are many different screening tests to choose from. The 2 most common screening tests are: first trimester screening, cell free DNA screening (also known as non-invasive perinatal screening or NIPS).  It can be hard to decide which one to choose! Your OB provider or genetic counselor can help you decide which test, if any, makes the most sense for you.

FIRST TRIMESTER SCREEN

The first trimester screen is done between weeks 11-14 of a pregnancy, but ideally it is done around 12-13 weeks. This test screens for Down syndrome and Trisomy 18. It involves an ultrasound that specifically measures a pocket of fluid at the base of a baby’s neck called the nuchal translucency or NT. Blood work is also done on the same day as the ultrasound to look at two pregnancy-related substances in the mother’s blood. This test will pick up approximately 85% of pregnancies that have Down syndrome; about 5% of patients who are told that they are at high risk will actually be normal. One benefit of this test is that it includes an ultrasound. There are other potential things that can be picked up on this ultrasound that are not specifically genetic abnormalities.

CELL FREE DNA TESTING (Non-invasive prenatal screening or NIPS)

Cell free DNA testing can be done any time after 10 weeks. This is a blood test where some of the baby’s blood is extracted from the mother’s blood. Genetic testing is then done on the baby’s blood. This test screens for Down syndrome, Trisomy 13, Trisomy 18 and can sometimes detect other chromosome conditions. It is also able to determine the sex of the baby.

This test picks up 99% of pregnancies that have Down syndrome. In addition, it picks up 92-99% of pregnancies with Trisomy 13 and Trisomy 18. In addition, this test is highly accurate. Less than 1% of cases are “falsely positive”.  There are times that not enough fetal cells can be found, leading to inconclusive results. This happens more often in women who are obese. Often, we can just repeat the test later when there are more fetal cells present.

Currently, the American College of Obstetricians and Gynecologists (ACOG) and the American College of Medical Genetics and Genomics (ACMG) recommend that cell free DNA screening be offered to all pregnant patients because it is the most accurate non-invasive screening test available.

Your obstetric provider or genetic counselor can discuss these screening options with you and help decide if genetic screening is right for you and your pregnancy.

Additional Resources for patients:

• Mayo Clinic Patient Education Materials (available in the Obstetrics Clinic)
• Mayo Clinic Guide to Healthy Pregnancy Book (Chapters 21-22)
• Genetic Support Foundation